[CUDA] Do not allow dynamic initialization of global device side variables.

In general CUDA does not allow dynamic initialization of
global device-side variables. One exception is that CUDA allows
records with empty constructors as described in section E2.2.1 of
CUDA 7.5 Programming guide.

This patch applies initializer checks for all device-side variables.
Empty constructors are accepted, but no code is generated for them.

Differential Revision: http://reviews.llvm.org/D15305

llvm-svn: 259592